This is a direct attack on competitors, designed to enlist the help of the federal government to raise payroll and put pressure on arch nemesis Walmart and any other company that has lots of retail other jobs that are not conducive to automation. Remember that Amazon thinks 5 years ahead, and they waited for the perfect PR and political threshold to announce these changes and capitalize on the news.
If you compare the UX at Walmart vs, say, Costco (which pays higher wages) I'm not so sure that paying the lowest is good business. i.e. is 1 Costco employee equal in productivity to 1 Walmart employee? I suspect not.
Walmart pays less because it has much lower profit margins. It serves far more customers and the average employee is a great deal less skilled. And it’s cheaper, with greater product choice. The average Walmart employee is definitely worse than the average Costco employee but Walmart is clearly the better business. It has higher gross profits, employs more people and serves more people.